Johannmeyer took part in the invasion of France, having been appointed leader of the regiment's 2nd Company on 1 April 1940.[2] In April 1941 he was transferred to the 14th Company of the regiment.[2] Subjected to the Army Group North, the regiment fought mainly at the Leningrad front, after the invasion of the Soviet Union (Operation Barbarossa). Because of his excellent leading skills, he was soon appointed commander of the 2nd Battalion on 1 April 1942.[2] There, for actions during the battles around the Lake Ilmen (May 1942), he was awarded the highest award of Nazi Germany to recognize extreme battlefield bravery or successful military leadership, the Knight's Cross of the Iron Cross on 16 May 1942; soon followed by his promotion to Hauptmann (1 June 1942).[2]

The 503rd Infantry Regiment was involved in the defensive battle of Nevel (south of Pskov Oblast, near Belorussia). In this sector, Johannmeyer distinguished himself with a particular action during the fighting in March 1943, and was thereby awarded the 329th[3] Oak Leaves (Eichenlaub) on the Knight's Cross on 18 November 1943.[2]

On November 25, his battalion attacked Soviet positions near the town of Sergeytsevo (northwest of Nevel) with the aid of the 502nd Heavy Tank Battalion in a forest. Otto Carius, a Tiger I tank commander of the battalion and one of the greatest tank aces of World War II, recalled on his memoirs that Johannmeyer was struck by sniper fire in the lungs from a Soviet marksman hidden in the foliage of a tree. Although it was initially thought that Johannmeyer's chances of survival were very slim, he managed to survive. Carius wrote that he was relieved to hear from Johannmeyer while he was in hospital in 1944. Nevertheless, Johannmeyer was absent from his official Oak Leaves awarding ceremony, as his condition was still utterly critical at that time.[3] In a few days' time, on 1 December he was promoted to major.[2]

On 1 March 1944, Johannmeyer was called to a training course for senior officer adjutants and three months later he was transferred to the OKH – Oberkommando des Heeres, Army High Command). From August 1944 onwards he served in the Army Personnel department (Heerespersonalamt), with the rank of Oberstleutnant i.G. (im Generalstab).[2][4]

By Hitler's order, Johannmeyer flew to Eastern Prussia to "clarify" the situation formed as the Soviets advanced to the Baltic States. As he always considered reports by army generals unrealistic, and refusing to accept that the Eastern Front was collapsing, he relied on his adjutants to receive "positive news". But, Johannmeyer, upon his return, reported that the army was in alarmingly bad state, especially after the formation of the Courland pocket and the pocket around Königsberg. Hitler refused to allow any means of withdrawal. When Johannmeyer referred to the civilian deaths due to the massive evacuation of eastern territories, Hitler replied in anger:[7]

“

I won't take this under consideration at all!

”

Johannmeyer was among the occupants of the Führerbunker, Hitler's underground headquarters in encircled Berlin. Johannmeyer was present at Hitler's last birthday ceremony on 20 April 1945.[8] During the night of 28–29 April, Hitler ordered that three copies of his political testament be hand-delivered to Field MarshalFerdinand Schörner in Czechoslovakia, Karl Dönitz in Schleswig-Holstein, and Paul Giesler in Tegernsee by Willy Johannmeyer, Bormann's adjutant SS-StandartenführerWilhelm Zander and Chief Press Secretary (Stellvertretender Pressechef) Heinz Lorenz, respectively.[9] The three officers said their farewell to Hitler and were handed a white dossier with the testament by Martin Bormann at approximately 4.00 am on 29 April. Armed with automatic weapons, wearing helmets and uniforms to break through Soviet lines, the officers left Berlin later that day.[10]

Oberstleutnant Graßmann took off with a Fieseler Storchliaison aircraft at around midnight, 1–2 May 1945 with the obligation to fetch Johannmeyer from Pfaueninsel, Wannsee and bring him to Field Marshal Schörner's headquarters, who was unaware of his appointment to Commander-in-Chief of the Army. However, Graßmann was unable to land in Berlin; he turned his Storch back and made an emergency landing in the Erzgebirge.[11]

[a] Regarding SS membership, the "Hitler Book" claims that he joined the SS in 1933. But, in contrast, Lexicon der Wehrmacht mentions no SS membership, and the ranks held clearly belong to the Wehrmacht - i.e. at Lexicon der Wehrmacht he is referred to as "Major", not the SS rank of Sturmbannführer. Additionally, Johannmeyer seems that have never served in an SS unit, but in the regular army instead, and wears no SS insignia in the photo provided. It is thereby uncertain whether Johannmeyer joined the SS, as "The Book Hitler" is often considered as unreliable source, but this possibility is not yet ruled out. The most plausible explanation is that he was transferred to the Wehrmacht in 1936, if we accept that he ever joined the SS.